Fixed iOS attempting to run Anroid unit tests

Summary: The logic that determined whether the Android unit tests needed to run was incorrect. Added an additional check around the entire Android unit test block in the mobile unit test engine to ensure that it doesn't get run unless there are actually Android tests to run.

Test Plan: Run `arc unit` on an iOS project with ArcanistMobileUnitTestEngine added. Ensure that no adb things try to run.

Reviewers: mbishop

Reviewed By: mbishop

CC: nrimer

Differential Revision: https://devportal.imobile3.com/D5708